This invention is concerned with soldering compositions.
Leadless component carriers are presently being used to increase component density on printed circuit boards as compared to that of leaded carriers. Leadless component carriers have electrical contacts nearly flush with the surface intended to be nearest the printed circuit board. It is known to be desirable to have this surface spaced somewhat (e.g., 10 mils) from the printed circuit board. This allows post soldering removal of flux residue and facilitates board cleaning. Furthermore, such a space tends to improve survival after thermal cycling.
Previous methods have used spacers or standoffs between the carrier and the board. One such technique is the placement of a dot of an organic material, such as an epoxy adhesive, under the carrier before soldering. Such materials, however, expand during thermal cycling, stressing the solder joint and increasing failure rates. Another method calls for placement of a removable spacer between the carrier and board, but this can cause missed solder joints because of a lack of contact between the component and solder paste, and requires hand soldering which is not cost-effective. Height and solder quality is operator sensitive.
Accordingly, an object of the invention is to provide a soldering composition suitable for soldering leadless component carriers to printed circuit boards which automatically spaces the carrier from the board.